The present invention relates to injection blow molding methods and apparatus in which a settable material is first injected into a mold cavity to form a parison and then the parison is subsequently expanded in a blowing operation to form an article such as a container. More particularly, the present invention is concerned with an injection blow molding method and apparatus that permits intricate detail of fine definition or resolution to be produced on the blown article.
Injection blow molding machines, such as disclosed in U.S. Pat. No. 4,076,484 having the same assignee as the present invention, are well known in the art for forming hollow articles, particularly containers of various sizes and shapes. Thermoplastic materials such as polypropylene or polyethylene are heated to a molten, semi-fluid state and are injected into the cavity of an injection mold to form a parison over a removable core or parison pin. Before the material has had an opportunity to completely set, the parison pin together with the parison thereon is removed from the injection mold and transferred to a blow mold where pressurized gas or air is blown through the pin to the interior of the parison and causes the parison to expand outwardly in the larger cavity of the blow mold. The expanded parison takes the shape of the blow mold cavity corresponding to a desired shape of the article, and is allowed to set sufficiently to retain that shape thereafter.
It is difficult to form articles in an injection blow molding process with intricate detail of high resolution such as narrow scuffrings, readable characters, designs and other functional and nonfunctional features. Due to the low temperatures that prevail in the blowing operation, the thermoplastic material assumes a more crystalline than amorphous character. With blown materials, fluidity and consequently the ability to form the materials with features of high definition are lost as temperatures drop during transfer from the injection mold to the blow mold. Additionally, the temperatures of the blow molds are controlled to cool the materials on contact and thereby accelerate the setting of the materials when the expanded parison assumes the shape of the mold cavity. Consequently, when contact occurs between the walls of the blow cavity and the outer surface of the expanded parison, the molding of detail on the exterior surfaces is not possible.
A further problem which aggravates the molding of detail features on a blown article is the fact that blow pressures are relatively low compared to pressures encountered in injection molding machines or in the injection side of an injection blow molding machine. Pressures in the order of 100-150 psi are typical for blow gases and air, and such pressures cannot be relied upon to press or extrude plastic material into corners or recesses of small dimension to provide a molded feature with sharp detail. In general, where the detailed feature has dimensions that are less than twice the wall thickness of the parison, molding with high resolution is not possible.
One prior art method of developing detail on the exterior of a blown article comprises an embossing process. An engraving bearing the desired detail is positioned in the bottom of a blow cavity, and the thermoplastic material is mechanically pressed against the engraving by means of the parison pin when the parison is inserted into blow mold. The parison pin may have an extendable tip for this purpose or the pin may have a length equal to the depth of the blow cavity to press the parison against the engraving upon insertion.
Quite obviously, the formation of detail by the above embossing process is limited to those situations in which the engraving is located directly below the parison pin at an area of the parison which does not expand during the blowing process. There are, however, many other situations in which intricate detail or molding features of high resolution are desired on the exterior of the finished article.
It is, accordingly, a general object of the present invention to provide method and apparatus for forming intricate detail on the article that is formed in an injection blow molding machine.